KSC AK74M system7two hop-up packing replacement

I disassembled a KSC gas blowback AK74M quite a while ago and then left it sitting around.

In my previous review, I mentioned that the hop-up packing was damaged and installed at an angle that caused poor trajectory, so I had planned to replace it. However, KSC had been out of stock of the part for a long time, so I left it as is (the hop-up packing was tilted).

However, with the re-release of the KSC AK74 gas blowback series in 2023, parts became available again, so I finally decided to get started on modifying it.

Let's get started by removing the barrel assembly.

Barrel removal

We will start with the field stripped (we will use the main body shown by the red dashed line).

Please refer to the previous field strip for disassembly steps up to this point. We will begin by explaining the handguard disassembly procedure.

First, lift the lever located near the rear sight (to release the handguard).

Lifting the lever will allow you to remove the upper handguard.

Next, looking from above at the front part that secures the handguard, you will see a small lever, so push the lever towards the muzzle.

Now the part that was holding the handguard in place will move, so slide it aside.

Now you can remove the lower handguard.

This removes the handguard, leaving it in the state shown in the next picture.

Next, looking at the receiver from the barrel side, you will see two Phillips head screws at the base of the barrel, so remove them.

• Screwdriver set
While cheaper options are fine, for affordable yet high-quality products, I recommend brands like Hazet, Tone, KTC, and Vessel.

Remove the two pins on the block where the rear sight is attached (see the location in the photo below).

Signet Pin Punch Set
It's not a luxury brand, but it's a recommended mid-range brand in the automotive industry that offers good value for money. Plus, it's inexpensive.

Eagle Force Maintenance Bench Block for Handguns
It's incredibly convenient to have. Moreover, it's designed for handguns but can also handle longer firearms, making it very useful. In fact, I personally didn't see the need for their M4 version.

Next, change your perspective and look for the grub screw on the top of the hop adjustment mechanism.

Remove this grub screw with an Allen wrench.

Now you can remove the barrel (inner barrel).

Next, remove the inner barrel and hop-up chamber from the outer barrel.

Remove the grub screws located near the hop-up adjustment dial (on both sides).

This allows you to remove the inner barrel and hop-up chamber.

These are all the parts I've disassembled so far. However, I've lost the grub screw that secures the outer barrel to the receiver.

Disassembly of the hop chamber

These are the removed inner barrel and hop-up chamber.

First, remove the inner barrel clip.

It's only held in place by the elasticity of the resin, so be careful not to break it.

Next, remove the grub screws shown in the picture below. There are two of these screws, one on each side.

As you remove the screws, the parts will come off, so be careful not to lose any as you disassemble it.

In particular, the hop-up packing's rubber bucking and plunger (yellow part) are small parts, so please be careful not to lose them.

Once you carefully remove the hop packing, the disassembly is complete.

This is a complete list of parts for the inner barrel and hop-up chamber.

Next, let's look at the inner barrel.

The barrel tip is tapered and fitted with an O-ring for positioning.

The barrel length is 217mm, which is quite short compared to the outer barrel. The barrel shape is unique to the KSC AK74, so I don't think there are any aftermarket parts available.

The hop-up window has a unique design, so Marui-type gaskets cannot be used.

The inner diameter is 6.02mm. This is a tighter fit compared to Marui's standard of 6.08mm.

Next, let's look at the hop packing.

This is KSC's unique bifurcated type, used since System7. Since adopting this type of packing, accuracy has improved considerably, and KSC products from around 2010 onwards are quite accurate.

Next, we'll replace the gasket with a new one and assemble it.

By the way, KSC offers a convenient online ordering service where you can purchase parts through their website, so please try it out if you have the chance.

KSC parts purchase site AK74M GBB system7 two

Hop chamber and inner barrel assembly

Before assembling, I noticed some burrs on the hop-up window of the inner barrel.

Remove this burr with a round file.

Diamond file set
These diamond files offer great value for money. The round files, in particular, are very useful. The 5-piece set is highly recommended.

This is the modified hop window section.

Since there is metal shavings, I will clean the inside of the inner barrel.

Now that the preliminary preparations are complete, we will assemble the chamber.

First, insert the sleeve into the inner barrel and apply silicone grease, avoiding the hop-up window as shown in the picture below. Applying silicone grease is more of a safety measure to ensure airtightness, so apply it at your discretion.

AZ Silicone Grease
This is a standard product from AZ. While Tokyo Marui's genuine grease is sufficient, the quantity is small, so AZ is recommended. It's also inexpensive.

Next, install the new hop packing.

Go through the hop dial.

Place the bucking rubber into the square recess of the hop packing.

Carefully insert the guide.

Place the plunger in the guide hole.

Carefully slide the hop adjustment dial towards the packing side and insert it.

Move the sleeve to the right side of the photo.

Finally, tighten the clip and the two grub screws to complete the assembly.

Check to make sure the hops are descending straight down in the completed chamber.

Incidentally, the original hop was tilted quite a bit, as shown in the photo below.

The chamber and inner barrel are now complete.

assembly

From here, we'll assemble it in the reverse order of disassembly.

First, insert the inner barrel and chamber into the outer barrel, then tighten one grub screw from the chamber side and two grub screws (on both sides) that secure the chamber.

Here are some important points to keep in mind when adjusting the hop.

First, let's look at the hop adjustment setting at 0 and the direction of the hop strength.

If you turn the dial from here to increase the hop, the dial will stop before reaching the maximum setting.

Turning the dial further from the state shown in the photo above is not recommended, as it can lead to the hop-up packing shifting or tearing. I suspect the tilt of the hop-up packing on my unit was caused by turning the dial too much.

Next, we secure the receiver to the outer barrel. This is fastened in a twisting direction using two pins, two screws, and one grub screw (I used a suitable grub screw because I lost the original), which provides considerable rigidity and appears to result in very high accuracy.

I'll skip the instructions for installing the handguard and just finish by assembling the bolt, return spring, and top cover.

As usual, I apply Bellhammer grease to the sliding parts of the movable components before assembling them.

Bellhammer No. 0 Gold Grease
This is a classic, top-quality grease. No. 0 is a low-viscosity grease. I generally use No. 0 for resin-to-resistance and resin-to-metal connections.

Bellhammer No. 2 Gold Grease
This is a classic, top-quality grease. No. 2 has the same ingredients as No. 0 but with a higher viscosity. I primarily use it for metal-to-metal connections.

This is completed.

Initial velocity and hop-up effect check

We'll measure the initial velocity while applying the hop-up to check how well it's working.

The conditions were the same as usual: the magazine was filled to the brim with HFC-134a, the surface temperature was 23°C (body temperature), and the ambient temperature was around 23°C.

The ammunition is VFC bio BBs, 02g.

First, let's look at the initial velocity with hop-up adjustment at 0.

The initial velocity is approximately 80 m/s.

Next, the initial velocity with half the hop adjustment amount.

The initial velocity is approximately 81-82 m/s.

Finally, here's the initial velocity with the hop-up adjustment at its maximum.

The initial velocity is approximately 83-85 m/s.

Many long gas blowback guns can see an increase in muzzle velocity when the hop-up is applied, similar to a velocity tune. The KSC AK74m I serviced this time now has a significantly increased muzzle velocity when the hop-up is applied, so the hop-up is working properly.

Also, before the maintenance, the hop-up packing was tilted, so when I applied strong hop-up, the BBs curved like a baseball curveball inside the outer barrel and I couldn't shoot properly, but now they come out properly, so the trajectory should be corrected as well (I plan to check it at the field later).

The work is now successfully completed.

Chronograph article

G&G Bio-Bullets 0.2g
It's inexpensive and accurate, so I use it for a wide range of purposes, from performance testing to airsoft games. It's been my favorite since it first came to Japan (around 2008). It lasts a long time for a biodegradable product.

G&G Bio-Bullets 0.25g
It's 0.25g. I generally use 0.25g BBs when playing airsoft.

Summary

The work itself wasn't particularly difficult; anyone could replace the hop-up packing as long as they had the right tools and parts.

If you feel that the trajectory of your KSC AK74m is strange, the cause is often related to the hop-up packing, such as deterioration of the packing over time, or the hop-up dial being turned too hard, causing the packing to tilt or tear. Trying these steps will likely solve the problem.

While KSC generally offers repairs for a fee, I think it's safest to ask them to do it. However, if you enjoy DIY, adjusting it yourself can be fun.

Upon disassembly, the external parts such as the handguard are quite realistic, including the mounting process. However, the outer barrel is fixed in a way that is unique to this airsoft gun, but it is securely and precisely fastened using many screws, so I think it will have high accuracy.

In terms of actual shooting performance, the muzzle velocity at 23 degrees Celsius with 0.2g BBs is quite high at 80-84 m/s (of course, within legal limits), and the hop-up packing is the KSC 2-way packing, which has had a good reputation for the past 10 years or so, so I think it will shoot quite well.

Thank you for your time.

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KSC AK74 series magazines
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KSC AKS74U Gas Blowback System 7two
This is a short model of the KSC AK74 featured in this article, with almost identical performance and structure. It's what's commonly known as a Krinkov.

KSC KTR-03 Gas Blowback System 7two
This is a KSC AK74 Tactical Model. I believe its structure and performance are the same as the AK74m.